In mining roadway or tunnel support, the anchor rod is the most widely-used and large-consumed support equipment. With the continuous deepening of mining depth, surround rocks of roadway always have a characteristic of large deformation of soft rock. The conventional anchor rod has a low extension rate, and is not capable of adapting to the characteristic of large deformation of roadway surround rock, which may cause the supporting failure of the anchor rod, and generate accidents such as tunnel roof falling, overbreak and so on. To solve the problem above, the Chinese patent application No. 201010196197.2 discloses an anchor rod with constant resistance and large deformation, which has a large extension rate, the extension of which can reach to 600 mm to 800 mm. However, the conventional tension meter of the anchor rod cannot satisfy the testing requirements of this kind of anchor rod.